Lost Garmin 510 white back. My stromlo

Rode down Skyline & the luge. Gave my son th Garmin probably lost it when he came off half way down the Luge. About 1 pm or just after
It's a 510 white backing
I have the serial number
I went up again to try to find it.
I know some people found it as they were asking around down at the car park . Bu by the time I came back down to the car park I was too late

Someone suggested some sites I should check?

Thanks in advance